The present invention relates to cryogenic sorption refrigeration systems. More specifically, the invention relates to two-stage sorption refrigeration systems.
Sorption refrigeration systems have been developed in the past to provide refrigerators with long lifetimes (e.g. 10 years or more) which will operate with little or no mechanical vibration.
Such refrigerators have been particularly useful in aerospace environments such as on board a space satellite where longevity of the refrigerator is of utmost importance and sensitive equipment must be cooled without vibrational disturbance.
Refrigeration systems employing non-mechanical compressors, generally referred to as sorption refrigerators, employ either a physical adsorption (physisorption) system or a chemical absorption (chemisorption) system.
A chemisorption refrigerator, providing cooling in the temperature range of 55.degree. to 100.degree. K. is disclosed in U.S. Pat. No. 4,697,425 to Jones. The refrigerant used in U.S. Pat. No. 4,697,425 is oxygen.
Briefly, in a chemisorption system such as the one disclosed in U.S. Pat. No. 4,697,425, a sorbent material reversibly chemically reacts with oxygen to absorb oxygen at a relatively low first temperature and pressure. When heated to a second temperature, the sorbent releases oxygen at high pressure. A container is provided to contain the oxygen while it is heated. Once an amount of oxygen is adequately pressurized an outlet valve provided on the container is opened to direct the pressurized oxygen away from the sorbent material. The oxygen is then subjected to precooling and directed to the high pressure side of a high pressure/low pressure orifice (Joule-Thomson) expansion valve. Heat is absorbed (and cooling takes place) when the pressurized oxygen expands into the low pressure side of the orifice. The oxygen at this point is typically in both gas and liquid phases at a low temperature. A liquid/gas transition chamber is provided for collecting the liquid oxygen. The liquid oxygen is used for cooling such as for the cooling of an infrared sensor. The liquid oxygen, by absorbing heat from the to-be-cooled material, will then boil and leave the transition chamber. On its return path to the containers for chemical absorption onto cooled absorbent material, the oxygen's temperature will be raised from the low temperature to the first temperature. By providing two containers and alternately heating and cooling the two containers, a continuous source of high pressure high temperature oxygen may be provided.
U.S. patent application Ser. No. 07/149,821 of Jones and Schember, discloses a physisorption-type refrigerator. This physisorption-type refrigerator provides cooling in the temperature range of 120.degree. to 160.degree. K.
The operation of a physisorption refrigerator such as disclosed in U.S. Pat. No. 4,831,829 can be summarized as follows. A refrigerant (in this case krypton) is precooled and physically adsorbed onto the cavity walls of a porous adsorption material, at a first temperature. Charcoal or another high surface area adsorbing matrice is generally used as the adsorption material. The sorbent (charcoal) and refrigerant (krypton) are then heated to a higher second temperature while the refrigerant is trapped in the sorbent. The pressure of the trapped refrigerant rises with the temperature and the refrigerant is driven off the sorbent surface. The refrigerant is then subjected to precooling and passed through a Joule-Thomson (J-T) expansion valve to decrease its pressure. The refrigerant at this point is typically both in liquid and gas phases at a low temperature. The liquid is collected into a liquid/gas transition chamber where the refrigerant begins reverting back to a gaseous phase by absorbing heat from a to-be-cooled heat source (refrigeration load). After leaving the transition chamber the temperature of the refrigerant is then raised from the low temperature to the first temperature and the refrigerant is readsorbed onto the cavity walls of the sorbent.
The disclosures of U.S. Pat. Nos. 4,697,425 and 4,831,829 are hereby incorporated by reference into this application.
Chemisorption refrigeration systems and physisorption refrigeration systems have different temperature ranges of operation and therefore may be advantageously employed together to provide cooling over a broader temperature range. A two stage refrigeration system using a methane physisorption upper stage to precool an oxide chemisorption lower stage is illustrated in FIG. 1. (This system is described in detail in Jones, J. A. and Blue, G. D., Oxygen Chemisorption Compressor Study for Cryogenic J-T refrigeration, AIAA 1558 (1987), incorporated herewith by reference). As shown in FIG. 1, a self-contained charcoal/methane upper stage 2 provides precooling to 140.degree. K. for a self-contained oxide chemisorption lower stage 4. As shown schematically in FIG. 1 the stages operate independently, in side-by-side fashion. Only the low temperature portion of the upper stage 2 is in contact with portion 6 of lower stage 4 to provide precooling of the oxygen.
One of the primary disadvantages of sorption refrigeration systems has been their unusually high power requirements when compared to conventional mechanical refrigeration cooling systems. A two-stage oxide/krypton sorption refrigerator could be expected to require about 155 watts of heat to produce 1 watt of cooling at 65.degree. K. This compares with about 60 watts of power for a mechanical refrigerator.